There is something to be said about fast food being more convenient then healthy food that helps you be thin. I will give you that. But wait, how about this. Convenient when? It is definitely convenient now. How about later, when you don't have enough energy to do anything but go to sleep? Or when you cannot focus because all you feel like doing is taking a nap. Or is it more convenient when you have indigestion and heartburn and don't feel like doing anything?
We live in a world of now. A world of cause and effect. But the problem is that most people only look at immediate cause and immediate effect. Not fully realizing the body does not work on an instantaneous process. If you put something in your mouth, it can take hours to process the food, even days if your system is not working so good. So when people get heart burn, or are tired, or get headaches they often miss the fact that it may have been something other than what happened immediately before. You cannot "save time".
You can only spend it wisely. And no matter how convenient it may seem at the time, food that is "fast" never saves you time in the long run. This is also a little trick that is interesting and works everywhere in your life. The solution to why it seems like there is never "enough time." It is because we falsely believe we are doing things that save time. Almost everything you do to "save time" only ends up costing you more time. If you eat fast food because you are in a rush, you will lose at least double the time later you supposedly saved.
How much time is wasted because you have no energy to do what you want at the end of the day? How much time is wasted because you don't feel like doing anything? How much time do you spend not feeling ready to go? How much time do you spend being unproductive because of brain fog? These are the type of things that your body does when you eat fast food to "save time". And these are only the semi-immediate effects.
Later in life your body starts doing all kinds of things that slow you down and don't allow you to do everything you want. This is in a large part due to what you put into your body. Because as you remember, if you try and build a brick house with rocks and stones, sooner or later it will fall apart.